1. Field of the Invention
The present invention relates to a broad band switching network focusing on an in-company use based on so-called broad band ISDN using ATM (Asynchronous Transfer Method) technology.
2. Description of the Related Art
Efforts for integrating individual service networks such as telephone networks, data networks, FAX networks, and so forth which have been developed and constructed over 100 years of history into one network system with ISDN (Integrated Services Digital Network) have been made throughout the world.
As the first step for constructing the ISDN system, narrow band ISDN systems have been operated in advanced countries including Japan since 1988. In addition, besides integration with a broadcasting network by using a broad band ISDN based on the ATM technology, the engineering developments of the ISDN network have been initiated by CCITT (International Telegraph and Telephone Consultative Committee) and promoted in major laboratories in the world.
Nevertheless, most of developments of the broad band ISDN and ATM engineering have been focused on the construction of public networks such as developments of office broad band ISDN switching equipments and so-called MAN (Metropolitan Area Network) including DQDB (Distributed Queued Dual BUS) which has been studied by IEEE 802 Standardization Committee and Owell Ring which has been developed by BTR Company, England. In other words, thus far, the developments of the ISDN network focusing on in-company use are small.
FIG. 46 is a schematic showing a long term demand estimation of the broad band ISDN in West Germany. The figure represents the relationship between the cost ratio of broad band ISDN against narrow band ISDN (horizontal axis) and the number of subscribers (vertical axis), respectively.
According to the figure, when the cost ratio becomes 3 or less, the ISDN will be rapidly spread out in general homes. When the cost ratio is higher than 3, the use of the ISDN will be limited to companies. In the broad band ISDN, ultra high speed subscriber network interface of 155.52 Mbps or 622.08 Mbps is supplied to general users by using latest technologies such as optical fiber cables as well as the ATM technology. However, at present time, the cost reduction of key components for achieving the broad band ISDN, such as optical communication parts and ultra high speed logical circuits (LSI) including ECL has not been satisfactorily accomplished in comparison with that of semiconductor memory devices. To satisfactorily accomplish the cost reduction, it would take more 10 years. It is estimated that the cost of CPU, memory, and CMOS logic circuits will be reduced into 1/16 in the forthcoming five years. In contrast, the cost of the key components of the broad band ISDN such as optical communication parts, ultra high speed logic circuits, and analog circuits will be reduced into 1/4 at most. In addition, such cost reduction of the key components of the ISDN is calculated by considering remarkable demand thereof as a precondition. Thus, unless the demand is strong, neither the cost reduction nor technology development would be accomplished.
Thus, it is necessary to accomplish the broad band ISDN service satisfying the needs in companies by means of the current technologies at a reasonable cost. In addition, if the satisfactory demand of company use is estimated by the accomplishment of the ISDN service, the engineering development and cost reduction of the above mentioned key components will be promoted and thereby the broad band ISDN service will be also acceleratingly spread out to general home users. In other words, to utilize and spread out a broad band ISDN, it is necessary to practically accomplish a private brunch switching system based on the broad band ISDN for the company use at a reasonable cost.
According to the related art, the private branch switching systems based on the broad band ISDN are roughly categorized as a centralized switching method, a distributed switching method with hierarchical network, and a LAN method.
In the case of the centralized switching method, with one private branch exchange (PBX), calls are centralized and switched. Thus, depending on the capacity of calls, various types of exchanges should be provided. Consequently, the accomplishment of the compatibility of software among the models (equality of services) becomes difficult (particularly, as the capacity of switch increases, functions being added by the users tend to become large). To maintain the compatibility, too much labor and cost would be required. In particular, the ATM switch which switches calls at an ultra high speed of 155.52 Mbps or 622.08 Mbps might be accomplished with a CMOS technology. However, the power consumption of the switch would become large (for example, even a small ATM switch of 8.times.8 would consume a power of 10 W; a large ATM switch of 1000.times.1000 would consume a power of 10 KW). In addition, countermeasures against heat radiation such as a heat pipe and high density heat exchanger would be required and thereby the cost would be increased. Moreover, when the large capacity model were redundantly constructed, the size of the equipment would become large like that of a super computer. It would prevent general company users from employing the equipment.
On the other hand, in the distributed switching method with a hierarchical network shown in FIG. 47, as exemplified with a so-called fourth generation PBX ("Front End of ISDN", "DENKI TSUSHIN (Telecommunication)", November 1989 and December 1989), a plurality of DSN's (Distributed Switching Nodes) 2 are connected to a higher rank network 1 composed of an optical loop network, each DSN being controlled so that the entire system operates as one exchange. Thus, since the number of DSN's can be increased and decreased when necessary, only one model of the equipment can cover a wide capacity range.
Nevertheless, to accomplish the distributed switching method with the hierarchical network, besides the development of distributed switching nodes, a high rank network should be developed and thereby the number of development steps would be remarkably increased. In addition, in the high rank network, the cost of stations 3 forming an ultra high speed optical loop network of 1.6 Gbps or 6 Gbps would become expensive. Thus, the capacity of the distributed switching nodes 2 should be large so that the cost of the stations 3 could be covered (that is, the cost of the stations could be ignored). Thus, beside a redundant construction of the distributed switching nodes 2, various countermeasures of reliability such as duplicate homing arrangement of the connections of the stations 3 should be taken. In such an arrangement, each DSN is connected to two stations, one of which is active. When a defect takes place in the active station, it becomes inactive and the inactive station becomes active. Thus, the entire system would become much expensive.
In the LAN method, for example, as shown in FIGS. 48 and 49, since an optical bus 4 of 155.52 Mbps is shared with a plurality of access units 5 and terminal equipments 6, the number of access units 5 and terminal equipments 6 connectable to the optical bus 4 is inevitably limited. In a large scale system, besides the same problem as the above distributed switching method, another problem of occurrence of a serious defect would take place if a plurality of access units 5 have defects. In addition, since the access units 5 forming the system are distributedly disposed at a user site, the maintenance work including power supply control becomes complicated. In particular, since optical connectors connecting optical fibers have been precisely produced, they might be often exposed to dust due to the user's operations and thereby a communication failure would frequently take place.
As was described above, when the broad band switching system focusing on the company use in the conventional method were constructed, the number of production steps would be increased and thereby the cost would be raised and a critical problem with respect to reliability and maintenance would arise. Thus far, practical countermeasure against the above mentioned problems have not been provided.